In digital communications systems, the signal to noise ratio (SNR) is used to predict the BER (bit error rate). In order to improve the signal to noise ratio, noise sources must be understood and the noise level minimized when possible. It is also important that the signal power to be maximized at the decision point for the bit under consideration.
A well known impairment that degrades this concentration of signal power at the decision point in fiber optic communications systems is dispersion. The dispersion effect can be explained if we assume that the transmitted signal can be represented as the sum of its components. Chromatic dispersion, polarization mode dispersion, and modal dispersion are the most common types of dispersion. These components cause small differences in propagation characteristics through the fiber. The received signal is affected by the sum of these components, resulting in inter symbol interference (ISI) by spreading the energy of each optical pulse over neighboring bits. The dispersion can thus cause bit errors in the receiver by confusing 1s and 0s.
Dispersion is present in all optical systems, but its effects become worse over longer spans and at higher transmission speeds. Long-haul systems already incorporate optical compensation elements to correct for chromatic and polarization dispersion compensation. A new alternative is electronic compensation. Electronic Dispersion Compensation (EDC) circuits have been proposed as a lower cost and lower power solution, see e.g. papers by A. Gandhi and S. Behtash, Electronic Dispersion Compensation, Santel Networks Inc., 2002; F. Buchali et al., Reduction of the chromatic Dispersion Penalty at 10 Gbit/s by Integrated Electronic Equalisers, Optical fiber Communication Conference 2000, volume 3, pp. 268-270; K. Azadet et al., Equalization and FEC Techniques for Optical Transceivers, Journal of Solid-State Circuits, Vol. 37, No. 3, pp. 317-327, March 2002; J. Lee and A. P. Freudorfer, MMIC Adaptive Transversal Filtering Using Gilbert Cells and is Suitable for High-Speed Lightwave Systems, IEEE Photonics Technology Letters, Vol. 12, No 2, February 2000; Meng-Lin Yu et al., An Arbitrary Fast Block processing Architecture for Decision Feedback Equalizers, Proceedings VLSI Technology Systems and Applications Conference 1999, pp. 175-178; and an article “Electronics holds the key to low-cost compensation”, FibreSystems Europe, December 2002, p. 15; in patent applications to Farjad-Rad entitled “Analog N-Tap FIR Receiver Equalizer” Pub. No. US 2001/0043649 published Nov. 22, 2001 and to Casper entitled “Equalization of a transmission line signal using a variable offset comparator, Pub. No. US 2003/0016091 published Jan. 23, 2003; and in certain marketing materials, e.g. description of “Eyemax” technology distributed by Applied Micro Circuits Corporation, and other marketing materials for electronic equalization distributed by BigBear Networks Inc., and for Electronic dispersion compensation engine distributed by Scintera Networks Inc.
A typical fiber optic communications system 10 with electronic dispersion compensation is illustrated in FIG. 1. Such a system includes a Transmitter 12, coupled to an electro-optic (E/O) converter 14, a fiber link 16, an opto-electrical (O/E) converter 18, an electronic dispersion compensation (EDC) receiver 20, and an electronic dispersion compensation (EDC) controller 22.
A digital bit stream 24 from the transmitter 12 is sent to the E/O converter 14. The output of the E/O converter 14 is an optical signal 26 to be transmitted over the fiber link 16. The output of the fiber link 16 is an optical signal 28, coupled to the input of the O/E converter 18.
The output signal of the O/E converter 18 is an analog signal 30. The EDC Receiver 20 receives the analog signal 30, and outputs a digital data signal 32 and a recovered clock 34. The digital data signal 32 may be coupled to an input of the EDC controller 22 (dotted line), the output of which is a set of control signals 36, coupled to a control input 38 of the EDC Receiver 20.
As described above, impairments caused by dispersion distort the signal transmitted by the fiber link 16. As a result, the analog signal 30 at the output of the O/E Converter 18 is not an exact replica of the digital bit stream 24 that was sent by the transmitter 12.
The purpose of the EDC Receiver 20 is to process the analog signal 30 into the digital data signal 32, to be an error free (as much as possible) representation of the original digital bit stream 24. The method used by the EDC Receiver 20 is generally based on the idea of reversing the impairment (dispersion) caused by the fiber link.
The article “Electronic Dispersion Compensation” by A. Gandhi and S. Behtash, Santel Networks Inc., 2002, illustrates this concept in some detail, FIGS. 1a and 1b of that reference showing a possible EDC Receiver. Main components of the EDC receiver of the prior art reference are a Feed Forward Equalizer (FFE) and a Decision Feedback Equalizer (DFE). The characteristics of both the FFE and the DFE depend on the setting of weight factors. These weight factors are included in the set of control signals 36 in FIG. 1.
The weight factors may be statically set by the EDC controller 22, or may be algorithmically derived, for example by analyzing the received digital data signal 32. The detailed functionality of the EDC controller 22 is outside the scope of this document.
While the concept of Electronic Dispersion Compensation including FFE and DFE has been proposed and described in a number of theoretically oriented papers, or in industry survey articles, the realization of practical receivers with Electronic Dispersion Compensation, especially when considering high data rates of 10 Gb/s and more, still require the development of new circuits that achieve this function economically while consuming as little power as possible.